The major histocompatibility system of the dog (DL-A) was studied in more detail by immunogenetical methods. Suggestive evidence for the existence of two closely linked DL-A (sub)loci was found. The definition of three alleles at the first sublocus was clear for both mongrel and beagle populations. Three additional specificities appear to recognize antigens of a second sublocus, at least in the beagle colony. Differences in the frequency of the described specificities were shown between the mongrel and the beagle populations.